Before you opened the motor, it is normal for the wheel to roll forward with low resistance. The clutch in the planetary plate engages when you spin the wheel in reverse. Is it possible that the gears were reassembled with a piece of debris hidden in the new grease that prevents one of the gears from spinning. My tub of bearing grease has been around for 30 years.. I'm lucky it's not full of dirt.

I bought this planetary 9 years ago for a Bafang SWXH motor, half expecting that I would melt the existing gears. Sanity ruled. I never melted them. The gears spin freely, but the keyway section only rotates in one direction. You might recheck that.